Since the work of Ans et al. (Ber. 45, 1845, 1912), it has been known that hydrogen peroxide reacts with aliphatic carboxylic acids to form percarboxylic acids according to a reversible reaction: ##STR1##
Given the instability of the peroxyacids, this reaction is usually performed at a low temperature. Under these conditions, the state of equilibrium is attained only after several hours of reaction, and this reaction time is prohibitive for an industrial procedure. Thus, it is necessary to use a catalyst. Only one type of catalyst has been proposed up until now: strong mineral acids, such as sulfuric acid, methanesulfonic acid, the arylsulfonic acids, phosphoric acid, the acid phosphoric esters, trifluoroacetic acid, as well as acid cation resins such as Dowex 50 and Amberlite IR-120.
This catalytic process has given rise to numerous studies (D. Swern, ORGANIC PEROXIDES, Wiley Interscience, 1970, Vol. 1, pages 313-369, and pages 428-439), from which it is clearly apparent that the first stage of the reaction is the protonation of the acid function, involving the formation of an oxonium structure capable of reacting with H.sub.2 O.sub.2, leading, after dehydration, to percarboxylic acid, as per the model: ##STR2##
Hydrogen peroxide is most often used in the form of commercial aqueous solutions containing 30-70% water. Moreover, the reaction also produces water, and the state of equilibrium is thus attained well before the hydrogen peroxide is fully transformed. Under these conditions, the product of the reaction is in effect a mixture of acid, hydrogen peroxide, per-acid, water, and strong acid. Because of this, the use of such a mixture as a means of oxidation in organic chemistry produces rather average yields.
To overcome this drawback, it has been proposed that the operation take place in the presence of a heavy excess of carboxylic acid, so as to shift equilibrium toward the right. In this way, by using 10 moles of acetic acid to one mole of hydrogen peroxide, one may obtain a conversion rate of 90% of the hydrogen peroxide into peracetic acid. Use of such an excess allows one to obtain only very diluted solutions of per-acid, and often involves losses in yields due to side reactions, without including the problems of subsequent separation of the products of the reaction.
The proposal has also been made, such as in U.S. Pat. Nos. 2,877,266 and 2,814,641, to operate only with a very slight excess of carboxylic acid, but to operate in the presence of a strong mineral acid and an azeotropic entrainer, in order to eliminate the water and thus shift the equilibrium (I) to the right. Such a practice is in fact excellent in terms of yield of percarboxylic acid in comparison to the hydrogen peroxide used. Compared with the preceding techniques, one could expect that this technique produces high yields in oxidation reactions in organic chemistry. This is scarcely the case, and the yield may be even worse, since the strong-acid catalyst very often gives rise to side reactions. For example, it is well known that, in reactions of epoxidation of olefins by per-acids, the epoxide formed is easily opened and transformed into a mono- or di-ester under the effect of strong-acid catalysts.
It is true that the strong acid may be advantageously neutralized, but then the corresponding salt is generally insoluble in the medium and poses separation problems which are not insignificant on the practical level. Sometimes, the salt is even as good a catalyst of side reactions as the acid itself.
This is why a method has been proposed recently, as in French Pat. Nos. 2,359,132 and 2,300,085, for preparation of organic solutions of percarboxylic acids in two stages, which consists of causing hydrogen peroxide (20-35% solution) to react with propionic acid in an aqueous solution containing 10-45% sulfuric acid, and then extracting the perpropionic acid with the aid of a solvent, such as benzene or dichloropropane. The aqueous phase must be concentrated in order to eliminate the water contributed by the H.sub.2 O.sub.2 solution and by the reaction. The organic phase is washed in order to eliminate H.sub.2 SO.sub.4, then dried by, for example, azeotropic distillation. This solution makes it possible in effect to obtain an organic solution of perpropionic acid that is anhydrous and free of sulfuric acid. However, this is a technique which is difficult to put into practice, and consequently costly.